1. Field of the Invention
The present invention is directed to the field of exhaust gas chemistry responsive sensors. More particularly, the present invention is directed to that portion of the above-noted field which is concerned with the construction of an exhaust chemistry responsive sensor for insertion in the exhaust system of an automotive internal combustion engine to provide a signal indicative of the air to fuel ratio of the combustion mixture producing the exhaust gases. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of an exhaust gas sensor which may be used to indicate a condition of stoichiometry in the combustion mixture which is generating the exhaust gases as a byproduct of combustion and which sensor may be used as an input device for an air/fuel ratio controller such that the combustion mixture may be maintained at stoichiometry. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with providing an exhaust gas chemistry responsive sensor which will be substantially lower in cost and of substantially less complexity, while being rugged and durable, than prior exhaust gas sensors.
2. Description of the Prior Art
There are generally speaking, two classes of exhaust gas sensors. Each makes use of a material which responds principally to the partial pressure of oxygen in the exhaust gases. The first of these, which is exemplified by the use of zirconia as the operative material, responds to a differential partial pressure of oxygen between a reference source of gas and a gas being sensed to generate a galvanic voltage or electromotive force between the surfaces of the material which are exposed to the two gases. This generated voltage may be used as a signal. These devices require that the surfaces exposed to the gases be provided with porous electrodes and that one surface be exposed to a relatively constant reference source while the second surface is exposed to the exhaust gases. This requirement presents constructional problems since it is normally the practice to use ambient air as the reference gas. This introduces substantial temperature gradients across the ceramic material. In order to provide rapid response times and for various other desirable operating characteristics, the zirconia material is preferably kept thin. The above-noted requirement and the preferred thinness also present sealing problems as well as other problems of an electrical nature. As a result, this class of devices tends to be fragile, expensive and relatively unreliable after being in use for a term of time less than that required to give an average of about 50,000 driving miles of service.
A second group of exhaust gas chemistry responsive sensors, which may be typified by the use of for example titania ceramic material as the operative material, exhibits an electrical resistance resistance which varies, at elevated temperatures, as a function of the partial pressure of oxygen in the gaseous environment of the ceramic and as a function of temperature. U.S. Pat. No. 3,886,785 describes a titania ceramic exhaust gas sensor configuration which utilizes an electrical heat source to provide the sensor with an initial heating and to thereafter maintain the sensor at a specific selected elevated temperature so that resistance variations will not be caused by fluctuations in the exhaust temperature.
Electrical heating means are typically provided in the form of an electrical resistance coil formed of platinum conductive wire. Such a heat source contributes substantially to the cost of a sensor, both from the standpoint of the cost of the platinum material and from the standpoint of the manufacturing complexity presented by the necessity of mounting the heater and communicating the heater, through the support ceramic material, to a separate electrical source for energization. Precise temperature control is required to eliminate temperature variations from influencing the sensor signal and to provide a very accurate temperature control particularly for operation of the associated internal combustion engine at nonstoichiometric combustion mixture ratios.
Investigation of the electrical resistance versus air/fuel ratio response curve of titania exhaust gas sensors has indicated that the resistance value of the titania varies substantially for the exhaust by-products of combustion mixtures which experience a lean to rich or rich to lean excursion or transition. In many instances, this variation may be several orders of magnitude, even in the face of adverse temperature variations. It is therefore an object of the present invention to provide a titania-based exhaust gas partial pressure of oxygen sensor to operate in the exhaust system of an internal combustion engine operated with a combustion mixture having a stoichiometric air/fuel ratio which is low in cost and relatively simple to assemble.
One continuing objective which the automotive industry in general has in fabricating any power train related component is maximum durability. The federal law has further stimulated the automotive industry to attempt to obtain, in the case of pollution control related engine components such as an exhaust gas sensor for use in a feedback air/fuel ratio control system, a durability factor which would be equivalent to operation of the average vehicle over approximately 50,000 miles. Under such a requirement, an exhaust gas sensor would be required to undergo a large number of thermal cycles and considerable vibration as well as being required to withstand the extremes of seasonal weather contaminates to which a vehicle may be subjected. Such a device, in order to be cost effective, would have to achieve the desired level of operation and reliability while maintaining as low a cost as possible. Since the sensor and its associated mechanical hardware would be subjected to the high temperature environment of the exhaust system and could be expected to be subjected to exposure to road salt and the like it would be necessary that the electrical portion of the sensor be capable of withstanding thermal cycling in the presence of salt environment. Conventional low cost means of thermal and environmental insulation would not normally be expected to hold up to this type of environment and the number of electrical leads associated with the exhaust gas sensor would multiply the statistical chances of failure. It is therefore a further and specific object of the present invention to provide an exhaust gas chemistry responsive sensor requiring only a pair of electrical leads which may be arranged in such a fashion as to assure maximum protection against salt, road spray, and splash. While various of these objectives have been achieved with the sensor construction according to the prior art and particularly with the above-noted copending, commonly assigned patent application Ser. No. 609,767 the reduction of cost without incurring performance or operational sacrifices is a continuing objective. It is therefore a further and specific objective of the present invention to provide a low cost, low complexity exhaust gas sensor of the variable resistance type to operate as a stoichiometry indicator in an exhaust gas feedback responsive air/fuel ratio controller. More particularly still, it is an object of the present invention to provide an exhaust gas sensor of rugged construction which is low in cost and which is of sufficiently simple construction that it may be manufactured on largely automated machinery.
The above-noted copending, commonly assigned patent application Ser. No. 609,767 sets forth the desirability of providing an exhaust gas sensor construction in which the electrical connection between the resistive type sensor material leads (the wafer leads) and the electrical connector which will communicate variations in sensor resistance as an input parameter to a feedback operated air/fuel ratio control fuel system be made at a location which is interior to the sensor device. According to the construction there described, a two-piece ceramic insert member is provided to allow convenient access to the interior of the sensor device so that the electrical interconnection may be conveniently and realiably accomplished. The sensor construction according to this prior art teaching has achieved significant cost savings by reducing the amount of precious metal, platinum, conductor to an absolute minimum in the sensor configuration but has, unfortunately, added a degree of complexity to the manufacturing operation occasioned by use of a two-piece ceramic insulator.
Since the reduction of components ordinarily carries with it a reduction in cost and manufacturing complexity, it is therefore a further and specific object of the present invention to provide an exhaust gas sensor construction which utilizes a one-piece ceramic insert member but which maintains the electrical interconnection between the sensor leads and the electrical terminal conductor at a point which is interior to the sensor device. More specifically, it is a specific object of the present invention to provide an exhaust gas sensor construction in which the electrical interconnection between the precious metal sensor lead and the electrical terminal conductors may be accomplished interiorly of the ceramic insert member.
Partial pressure of oxygen responsive sensors of both classes have required the provision of shield means to prevent the direct impingement of the exhaust gases upon the sensor material. This is necessary since pressure pulsations exist within the exhaust gas and the pulsations would quickly produce fractures within the thin ceramic material if directly exposed to the exhaust gas stream. According to the prior art, an apertured or perforated cylindrical body of corrosion resistant material is arranged to extend from the housing or outer body. This shield member is typically formed of a metal such as stainless steel or is a simple extention of the metallic outer body. In either event, the shield member has limited useful life due to the highly corrosive atmosphere of the exhaust gas and adds expense to the cost of the sensor device. It is therefore an object of the present invention to provide nonmetallic shield means for the sensor wafer.